Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
Med Phys. 2013 Dec;40(12):121909. doi: 10.1118/1.4829513.
Fluence field modulated computed tomography (FFMCT) presents a novel approach for acquiring CT images, whereby a patient model guides dynamically changing fluence patterns in an attempt to achieve task-based, user-prescribed, regional variations in image quality, while also controlling dose to the patient. This work aims to compare the relative effectiveness of FFMCT applied to different thoracic imaging tasks (routine diagnostic CT, lung cancer screening, and cardiac CT) when the modulator is subject to limiting constraints, such as might be present in realistic implementations.
An image quality plan was defined for a simulated anthropomorphic chest slice, including regions of high and low image quality, for each of the thoracic imaging tasks. Modulated fluence patterns were generated using a simulated annealing optimization script, which attempts to achieve the image quality plan under a global dosimetric constraint. Optimization was repeated under different types of modulation constraints (e.g., fixed or gantry angle dependent patterns, continuous or comprised of discrete apertures) with the most limiting case being a fixed conventional bowtie filter. For each thoracic imaging task, an image quality map (IQMsd) representing the regionally varying standard deviation is predicted for each modulation method and compared to the prescribed image quality plan as well as against results from uniform fluence fields. Relative integral dose measures were also compared.
Each IQMsd resulting from FFMCT showed improved agreement with planned objectives compared to those from uniform fluence fields for all cases. Dynamically changing modulation patterns yielded better uniformity, improved image quality, and lower dose compared to fixed filter patterns with optimized tube current. For the latter fixed filter cases, the optimal choice of tube current modulation was found to depend heavily on the task. Average integral dose reduction compared to a uniform fluence field ranged from 10% using a bowtie filter to 40% or greater using an idealized modulator.
The results support that FFMCT may achieve regionally varying image quality distributions in good agreement with user-prescribed values, while limiting dose. The imposition of constraints inhibits dose reduction capacity and agreement with image quality plans but still yields significant improvement over what is afforded by conventional dose minimization techniques. These results suggest that FFMCT can be implemented effectively even when the modulator has limited modulation capabilities.
通量场调制计算机断层扫描（FFMCT）提供了一种获取 CT 图像的新方法，通过患者模型动态改变通量模式，试图实现基于任务、用户规定的区域图像质量变化，同时控制患者的剂量。本研究旨在比较 FFMCT 在调制器受到限制的情况下，应用于不同的胸部成像任务（常规诊断 CT、肺癌筛查和心脏 CT）的相对有效性，这种限制可能存在于实际应用中。
为模拟人体胸部切片定义了一个图像质量计划，包括每个胸部成像任务的高、低图像质量区域。使用模拟退火优化脚本生成调制的通量模式，该脚本试图在全局剂量限制下实现图像质量计划。在不同类型的调制约束（例如，固定或机架角度相关的模式、连续或由离散孔径组成）下重复优化，最具限制的情况是固定的传统蝴蝶结滤波器。对于每个胸部成像任务，预测每种调制方法的代表区域变化标准偏差的图像质量图（IQMsd），并将其与规定的图像质量计划以及均匀通量场的结果进行比较。还比较了相对积分剂量测量值。
与均匀通量场相比，FFMCT 产生的每个 IQMsd 在所有情况下都显示出与计划目标更好的一致性。与优化管电流的固定滤波器模式相比，动态变化的调制模式可提高均匀性、改善图像质量并降低剂量。对于后者的固定滤波器情况，发现优化管电流调制的最佳选择严重依赖于任务。与均匀通量场相比，平均积分剂量减少范围从使用蝴蝶结滤波器减少 10%到使用理想调制器减少 40%或更多。
结果支持 FFMCT 可以实现与用户规定值一致的区域变化的图像质量分布，同时限制剂量。施加限制会抑制剂量减少的能力和与图像质量计划的一致性，但仍比传统的剂量最小化技术提供的改善更大。这些结果表明，即使调制器的调制能力有限，FFMCT 也可以有效地实施。
